1. Field of the Invention
The present invention relates to the field of helicopter power systems and, in particular, to helicopters designed for fast forward flight or noise control. Specifically the invention relates to helicopter rotor systems designed to operate at a plurality of rotor speeds while the engine speed remains constant.
2. Description of the Prior Art
Prior art helicopters are subject to two main disadvantages. Firstly, conventional helicopters are able to only operate effectively at high noise levels. The noise is rotor-generated noise and not necessarily engine-generated noise. In dense urban areas under domestic conditions, the noise performance levels of helicopters is subject to increasing concern and regulation. In some cases noise levels are so objectionable that helicopter flight in certain urban areas is prohibited. Moreover, in combat conditions the high noise levels inherently characterizing a high-performance rotor system prevents the attach helicopter from operating silently and providing a short range suprise approach.
Secondly, helicopter power plants in general and, in particular, high performance turbine power plants, are designed to run optimally at a single RPM. Although turbine RPM can be varied by limited amounts, such variations are always at substantial trade-offs in performance.
Therefore, the flight performance or envelope of the helicopter, including its noise performance, is largely fixed to a practical degree by the fixed RPM performance from the power plant and rotor system. Therefore, what is needed is some means wherein the rotor system, including both the main and tail rotor of a helicopter, can be operated in a flexible manner using conventional power plants characterised by substantially constant RPM outputs to vary both flight envelope and rotor noise performances in an arbitrary manner.